The present invention relates generally to dielectric filters, and, more particularly, to a dielectric filter which may be surface-mounted upon a substrate, such as a printed circuit board.
A filter circuit is operative to generate a filtered signal responsive to application of an input signal thereto. Component portions of the input signal which are of frequencies beyond a pass band of the filter circuit are filtered by the filter circuit; such component portions of the input signal do not form a portion of the filtered signal generated by the filter circuit. Conventional filter circuits include, for example, band pass, band reject, low pass, and high pass filter circuits. One or more filter circuits typically comprise a portion of an electrical circuit.
While filter circuits may be comprised of any of many varied types of materials, filter circuits are generally classified according to the type of material utilized to construct the filters. More particularly, filter circuits may be comprised of either active or passive filter components. Accordingly, a filter circuit is generally classified as forming either an active filter or a passive filter.
An active filter component may be advantageously embodied in an integrated circuit, but an active filter circuit formed of such active components is generally linear over only a limited dynamic range. Accordingly, an active filter circuit comprised of active filter components exhibits proper filter characteristics over only the limited dynamic range in which the active filter circuit is linear.
Passive filter circuits comprised of passive filter components, e.g., combinations of conventional, discrete resistors, capacitors, and inductors are, conversely, linear over a greater dynamic range. The resistive, capacitive, and inductive component values of the passive filter components, and their electrical connections therebetween, define filter characteristics of the filter circuit formed therefrom. The passive filter components comprising the passive filter circuit may be connected in manners, and may be of resistive, capacitive, and inductive values, to form a filter circuit, such as any of the above-listed filter circuits. Because such passive filter circuits are linear over the greater dynamic range, passive filter circuits are utilized when proper filter characteristics are desired over the greater dynamic range. In many instances, therefore, filter circuits are comprised of passive filter components to form passive filters.
When a filter circuit forming a portion of an electrical circuit is positioned in a series connection with the electrical circuit, component portions of an input signal applied to the filter circuit within a resonant frequency of the filter circuit, are passed by the filter circuit. The resonant frequency of a filter circuit is determined by the component values of the components forming the filter circuit and their respective electrical connections therebetween. By appropriate selection of the component values of the components of the filter circuit as well as their electrical connections therebetween, component portions of an input signal applied to the filter circuit of frequencies within any desired range of frequencies may be passed by the filter circuit.
When a filter circuit forming a portion of an electrical circuit is positioned in a shunt connection with the electrical circuit, component portions of an input signal applied to the filter circuit within the resonant frequency of the filter circuit are shunted to ground by the filter circuit. By appropriate selection of the component values of the components of the filter circuit as well as their electrical connection therebetween, component portions of an input signal applied to the filter circuit of frequencies within any desired range of frequencies may be shunted by the filter circuit.
Electrical circuits forming radio receivers, radio transmitters, and radio transceivers all include filter circuits forming portions thereof. For instance, filter circuitry is utilized in radio receiver circuits to tune the receiver, and to filter a signal of unwanted signal portions generated during down conversion and/or demodulation of a signal received by the receiver.
Dielectric filters, comprised of ceramic and other dielectric filter components, are oftentimes utilized to form passive filter circuits. When formed of the ceramic material, such filter circuits are generally referred to as ceramic block filters due to the geometrical configuration of most of such filters. Such ceramic block filters are advantageously utilized to form portions of an electrical circuit as a single ceramic block forms the filter circuit. Typically, one or more cavities are drilled or otherwise created to extend through the ceramic block, and the side walls defining the cavities are coated with an electrically-conductive material. Surface portions of the ceramic block are also typically coated with the electrically-conductive material. Once coated with the electrically-conductive material, the cavities form resonators of characteristics defined by the lengths and the surface areas of the side walls which define the resonator. U.S. Pat. Nos. 4,431,977; 4,742,562; 4,673,902; 4,703,291; and 4,716,391 all disclose such ceramic block filters.
Care must be exercised to ensure that the ceramic block filter is properly connected to the electrical circuit. In some instances, coaxial cables are soldered to portions of the ceramic block filter to effectuate such connection. In other instances, such as that disclosed in U.S. Pat. No. 4,673,902, the ceramic block filter is coupled directly to the electrical circuit by a direct, solder connection. Because the ceramic block filter is susceptible to movement when the solder liquefies during solder reflow operations, such existing manner of connecting the ceramic block filter to an electrical circuit can, however, result in inadequate electrical connection of the ceramic block filter to the electrical circuit.
U.S. patent application Ser. No. 577,172 (now U.S. Pat. No. 5,045,824), filed on Sep. 4, 1990, and entitled, "Dielectric Filter Construction," discloses a ceramic block filter mountable upon a multi-layer circuit board having a mounting surface which matingly engages with a corresponding mating area formed on a top layer of the multi-layered circuit board. Problems associated with movement of the ceramic block filter occurring during solder reflow operations are obviated as the mated engagement of the ceramic block with the circuit board prevents movement of the ceramic block.
A dielectric filter construction mountable upon any type of circuit board which is similarly not susceptible to movement during solder reflow operations would be advantageous.
Care must also be exercised during design of electrical circuits which include such ceramic block filters as such ceramic blocks are conventionally mounted upon circuit boards. When circuit boards are stacked above one another, or when a single circuit board is housed within an enclosed housing, spacing of lengths at least as great as the heights of the ceramic blocks is required to permit positioning of the ceramic block upon the circuit board.
Disclosed in U.S. Pat. No. 5,023,580 is a dielectric block filter mountable upon a circuit board wherein the filter extends through an opening formed through the circuit board. The filter is supported in position by a bracket. As a portion of the ceramic block rests beneath the top surface of the circuit board, the spacing required above the circuit is reduced.